This invention relates to surface acoustic wave devices and to a substrate of lead potassium niobate, Pb.sub.2 KNb.sub.5 O.sub.15, (PKN) for use therewith.
ST-cut quartz is often utilized as a piezoelectric substrate material for a wide variety of surface acoustic wave devices (SAW) such as filters, delay time encoders, decoders, correlators and other signal processing devices. Unfortunately, ST-cut quartz possesses a low piezoelectric coupling constant and, therefore, is not suitable for use in SAW devices designed to have low insertion losses and broad bandwidths. As a consequence, a considerable research effort has evolved in an attempt to find other materials for use as SAW substrates that possess a high piezoelectric coupling constant and are temperature compensated. In attempting to find such materials, it has been discovered that in order to be temperature compensated, a material often possesses either a positive temperature coefficient of an elastic constant or a negative coefficient of thermal expansion. That such a concept is valid has been demonstrated by the results of recent calculations of the SAW properties of berlinite (which has a positive temperature coefficient of an elastic constant) and .beta.-eucryptite (which has a negative coefficient of thermal expansion). These calculations showed that both materials are indeed temperature compensated and have larger piezoelectric coupling constants than ST-cut quartz.
However, berlinite and .beta.-eucryptite still fail to possess a piezoelectric constant as large as is desired for certain SAW applications and, in addition, lack the low temperature coefficient of time delay and small electromechanical power flow angle parameters also desired for SAW applications. In attempting to find still newer materials which might prove useful and desirable for SAW applications, it was discovered that lead potassium niobate (PKN), which occurs in the tungsten bronze structure and belongs to the orthorhombic crystal class mm.sub.2 (C.sub.2v) is attractive for SAW applications. The most significant feature of the particular material of this invention is that its piezoelectric coupling constant is up to 12.6 times as large as that of ST-cut quartz. In addition, the diffraction spreading of PKN is less than that of an isotropic material, an attractive feature not shared by either quartz or berlinte.
Calculations undertaken during the research effort have shown that a particular orientation of PEN having the Z-axis in the plane of the plate and perpendicular to the direction of preparation provides a high piezoelectric coupling constant 12.6 times as large as that of ST-cut quartz. The particular crystallographic orientation of this invention for a Z-axis cylinder orientation is defined by the Euler angle: Lambda=74.4.degree.; Mu=90.0.degree.; and Theta=0.0.degree..
Currently, lithium niobate (LiNbO.sub.3) is used as a substrate material for surface acoustic wave devices which require greater bandwidths (for a given amount of insertion loss) than that obtainable with ST-cut quartz. But, because LiNbO.sub.3 has a large sensitivity to temperature, bulky and costly ovens are required for temperature control. This new crystallographic orientation of lead potassium niobate will make it possible to build SAW devices with far greater bandwidths than that possible with quartz, and with greater temperature stability than that possible with lithium niobate.
The most important feature of the crystallographic orientation of this invention is that its piezoelectric coupling coefficient is about 12.6 times as large as that of ST-Cut quartz. This makes it possible to build low insertion-loss SAW devices with low temperature sensitivity and larger bandwidths than those obtainable in the devices currently being built on ST-cut quartz.